Collaborative Research: Optical Transitions in Metallic Nanoclusters at High Pressure

Northwestern University, Evanston IL

Investigators

Abstract

NONTECHNICAL SUMMARY This research program contributes to innovations in nanomaterials that are useful in biomedical therapies, sensing, and catalysis. Nanoclusters that consist of fewer than ~200 metallic atoms are of interest due to their luminescence in near-infrared light, bio-compatibility and ultra-small size. The relationship between the structure and optical properties of these nanoclusters remains unclear, which prevents the design of nanoclusters with optimized performance. This program uses pressure to modify the structure of nanoclusters, in order to learn about the electronic transitions that are responsible for light absorption and luminescence, as well as the stability of nanoclusters under external stimulus. This research is closely tied to our educational plans through the training of postdocs and graduate students, by supporting underrepresented groups at the K-12 and college level in science and technology, and by supporting science teachers in the community. In addition, modules that explain the influence of mechanics on optical behavior will be incorporated into undergraduate courses during this program. TECHNICAL SUMMARY The proposed research is concerned with characterizing the properties of atomically precise clusters composed of gold or silver plus ligands using diamond anvil cell techniques to apply high pressure, and with optical measurements (both absorption and emission) to study excited states. The experiments will be modeled using electronic structure theory, including ground state calculations to determine structures, and excited state studies to determine optical properties, and including methods development based on second linear response theory and domain-separated density functional theory. The research is aimed at identifying optical properties such as plasmon to exciton transitions, and an increase in luminescence efficiency with increasing pressure; both properties are of interest to using the clusters in practical applications. The research includes three research objectives: (1) study of energy transfer responsible for broad absorption & near-infrared luminescence of the cluster material under pressure, (2) determine the source of anomalous multiple emissions of the nanoclusters, and (3) studies of 2-3 nm nanoclusters to characterize the transition from plasmonic to excitonic states. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.